A hard disk drive 10, as illustrated in FIG. 1, comprises a platter 12 constructed of magnetic material for storing information, in the form of data bits, for processing by a computing or processing device. The information is stored on the platter 12 by magnetizing small magnetic domains that retain the magnetization and thus can be magnetized to store a zero data bit or a one data bit. A motor (not shown in FIG. 1) spins the platter 12 (typically at speeds of 3,600 or 7,200 revolutions per minute) allowing a read/write head 14 to write data to or read data from the platter 12 as the read/write head 14 travels over the surface of the platter 12. The read/write head 14 does not make physical contact with the platter 12.
The read/write head 14 is affixed to an arm 16 controlled by a positioning mechanism 18 for moving the arm across an upper surface of the platter 12, between an edge 24 and a hub 26. Data bits are stored on the platter 12 in sectors 30 on concentric tracks 32. Typically, a sector contains a fixed number of bytes (for example, 256 or 512). A plurality of sectors are commonly grouped together into a cluster.
As illustrated in FIG. 2, to increase storage capacity a hard disk drive typically comprises a plurality of parallel platters 12A, 12B and 12G Read/write heads 14A through 14F write data to and read data from a top and bottom surface of each of the platters 12A, 12B and 12C The depiction of three platters and six read/write heads illustrated in FIG. 2 is merely exemplary.
The positioning mechanism 18 conventionally employs a high-speed linear motor or a voice coil motor to move the arm 16. In the voice coil embodiment, the voice coil is located adjacent to a magnet, which together operatively define the voice coil motor of the positioning mechanism 18. The hard disk drive 10 further comprises a controller (not shown) for providing current to excite and control the voice coil motor of the positioning mechanism 18. The excited voice coil motor rotates the arm 16, moving the head 14 across the surface of the platter 12 along an arc.
Data bits are written to and read from the hard disk drive 10, utilizing a magneto-resistive transducer as a sensing and writing element within the read/write head 14. The voice coil motor moves the arm 16 to a desired radial position on the surface of the platter 12, after which the head 14 electromagnetically writes data to the platter 12 or senses magnetic field signal changes to read data from the platter 12. The arm 16 is shaped and controlled such that it “flies” over the surface of the platter 12 as the latter rotates beneath it. Contact between the head 14 and the platter 12 is not desired.
Conventional transducers comprising the read/write head 14 employ a magnetically permeable core coupled with a conductive coil to read and write data on the surface of the platter 12. A write operation is typically performed by applying a current to the coil, thereby inducing a magnetic field in the adjacent magnetically permeable core. The magnetic field extends across the air gap between the head 14 and the platter 12 to magnetize a small region of magnetic domains to store the data bit. Information is read from the platter 12 when the magnetized region induces a voltage in the coil. Alternatively, reading can be performed using a magneto-resistive sensor, where the resistance varies as a function of the proximate magnetic field.
To increase the amplitude (and thus the signal-to-noise ratio) and the detection accuracy of the data bits as they are read from the platter 12, the head 14 is positioned as close to the platter 12 as possible. However, the low amplitude voltage signals produced in the head 14 during the read operation typically exhibit a low signal-to-noise ratio. Also, the high frequencies involved in the read operation tend to increase noise in the voltage signal. It is advantageous to improve the signal-to-noise ratio of the read signal to improve the accuracy (i.e., reduce the error rate) of data bit detection.
Known techniques for increasing the signal-to-noise ratio have focused on increasing the signal level and reducing noise in the head output signal, thus reducing noise effects that must otherwise be accommodated during the subsequent signal processing. Error detecting/correcting codes can be appended to the data words to account for noise effects. However, this technique increases the total number of bits (i.e., data bits plus error detecting/correcting bits) required to store information on the hard disk drive 10 and thus reduces the effective hard disk drive capacity. So called “giant magneto resistance detectors” generally produce a higher output voltage and thus have a higher signal-to-noise ratio than the inductive coil described above. Also, as the materials comprising the magneto-resistive device are improved to generate less noise during the reading process, the signal-to-noise ratio improves. Certain regions of the noise spectrum can also be filtered from the read signal using spectral filters. However, noise voltage remains in the spectral region processed by the signal processing circuitry to detect the data bits.